TEHRAN (Tasnim) – The UN’s Syria envoy applauded a "significant drop" in violence through the first 24 hours of a fragile ceasefire but said security concerns meant aid convoys stayed on hold.

The truce brokered by Russia and the United States began at sundown Monday, in the latest bid to end a conflict that has killed more than 300,000 people since March 2011.

The agreement aims to bring an end to fighting between Syrian President Bashar Assad's forces and a wide range of militants but excludes terrorists groups like Daesh (ISIL).

UN envoy Staffan de Mistura said on Tuesday reports reaching his office indicated "a significant drop in violence," in a rare respite in Syria's devastating conflict.

De Mistura noted isolated reports of conflict persisted, especially Monday night, but that by sunrise Tuesday the broad picture was positive.

The envoy said he had "no information about any UN trucks moving at this stage," demanding "assurances that the drivers and the convoy will be unhindered and untouched."

In second city Aleppo, AFP correspondents in both the rebel-held east and the government-held west reported nearly 24 hours had passed without airstrikes or rocket fire.

AFP correspondents in government-held Damascus and its rebel-controlled suburbs reported quiet too, with residents taking advantage of the lull in violence to mark the Eid al-Adha Muslim holiday.

Damascus accused the opposition of a series of violations, with its ally Moscow saying Syrian government forces were fully respecting the truce but that rebels had violated it 23 times.

"Syrian government troops have completely stopped firing" except in extremist-held areas, but "the same cannot be said for armed units of the moderate opposition controlled by the US," said Viktor Poznikhir, a senior Russian military officer.